1. Field of the Invention
The present invention relates to a reflector antenna used for radar, etc. which has improved characteristics by employing a subreflector.
2. Description of the Prior Art
Heretofore, a composite curves reflector antenna having composite curves such as a doubly-curved reflector on the main reflector surface have been known.
As illustrated in FIG. 1, the conventional composite curves reflector antenna generally comprises a main reflector 1 and a primary radiator 2. The aperture of the primary radiator 2 is covered with a dielectric cover 3.
During transmission, the radiation beam radiated from the primary radiator 2 is reflected by the main reflector 1, a shown by the broken line, to radiate in space.
As shown in FIG. 2, the surface of the main reflector 1 of the antenna has composite curves composed of a central sectional curve 1a and transverse curves 1b.
The central sectional curve 1a is selected by the shape of the beam in the vertical plane of the beam.
The curves 1b being transverse to the central sectional curve 1a are parabolic and have a focus point which is at the phase center F of the primary radiator 2.
In such an antenna, the focal distance to the diameter of the aperture of the main reflector 1 cannot be too long such that a broad beam should be provided from the primary radiator 2.
Accordingly, the primary radiator 2 providing such a beam should have an aperture having a diameter corresponding to substantially 1 - 3 wavelengths. When the primary radiator 2 is used for a high frequency band, such as the millimetric wave band, then the aperture of the primary radiator 2 should be quite small.
Accordingly, under such conditions, when a drop of rain or a piece of snow is deposited on the dielectric cover 3 covering the aperture of the primary radiator 2, the effect is quite high, whereby the conventional reflector antenna can not be operated for the millimetric wave band or higher.
Moreover, in the conventional reflector antenna, the length of the aperture of the main reflector 1 in the vertical plane is different from that of the horizontal plane. Accordingly, the angular aperture of the main reflector in the vertical plane should be different from that of the horizontal plane. Thus, only horns having different diameters in the vertical plane to that in the horizontal plane could be employed as the primary radiator.
The present invention is to overcome the above-mentioned disadvantages.